Process for the production of polyethers derived from oxetanes

ABSTRACT

Hydroxyterminated prepolymers, used in curing reactions with isocyanates to produce elastomers, are prepared by continuously adding over periods of 12-50 hours an oxetane monomer to a reaction mixture containing a diol initiator and a catalyst capable of catalyzing the cationic quasi-living cationic polymerization of the monomer, the rate of addition to maintain stoichiometric excess over the monomer. Polymerization is terminated by adding brine. Polymers of increased molecular weight and reduced impurity levels result.

The present invention relates to the production of polyethers derivedfrom oxetanes using cationic polymerization initiators. The polyethersare an important class of polymers finding application as detergents,disinfectants, absorbents and elastomer prepolymers amongst other uses.For many of these uses it is highly desirable to produce a pure productof controlled molecular weight and controlled polydispersity.

The cationic polymerization of oxetanes involves the opening of theheterocyclic oxetane ring under catalytic conditions. This mechanism ofpolymerization is described in U.S. Pat. No. 4393100 (Manser). Thecatalyst described by Manser combines with a preinitiator precursor toform an adduct capable of forming a cationic initiator with the oxetanemonomer. Polymer chains are built up as oxetane molecules add on to thereactive end groups of the initiator molecules, the number of suchchains being proportional to the number of preinitiator precursormolecules present.

In the process described in U.S. Pat. No. 4393199, the preinitiatorprecursor followed by the catalyst are added to bulk solutions of theoxetane. In some examples, the precursor is first added to the catalystto form the adduct which is then added to the bulk solution of monomer.Principal examples given of the precursor and catalyst are,respectively, 1,4-butane diol (a difunctional alcohol) and borontrifluoride etherate, the diol replacing the ether to give the activeadduct. A catalyst-to-diol molar ratio of at least 1.5:1 was foundnecessary to polymerise the monomer, whereas at a ratio of 3:1 andhigher, loss of polymer molecular weight control occurred and thepolydispersivity of the product become much higher. However, within thepreferred catalyst-to-diol molar ratio of from 1.5:1 to less than 3:1,the yield of polymer was only 63-68% when using 1,4-butane diol as theprecursor, indicating the presence of significant amounts of impuritiesin the product.

A further example of this process of polymerization as applied to3-nitratomethyl, 3-methyloxetane (NIMMO) is provided in a technicalreport by Morton Thiokol (Defence Technical Information Centrepublication No. 85837, Defence Logistics Agency, Cameron Station,Alexandria, Va., page E4 to E5.) This document relates to a bulkreaction polymerization of NIMMO using the methods of U.S. Pat. No.4393199 and provides a product containing about 25% impurity.

The present invention seeks to provide an improved process for theproduction of polyethers by quasi-living cationic polymerization ofoxetanes which provides products of increased reproducibility, reducedimpurity levels and more complete control of molecular weight.

Accordingly, the present invention provides a process for thepolymerization of oxetanes monomers comprising the steps of (a) mixingtogether a catalyst capable of catalysing the cationic polymerization ofthe monomer with a preinitiator precursor to yield an active adduct ofthe catalyst and precursor, (b) bringing the adduct into contact withthe monomer so as to cause the adduct to form an initiating species withthe monomer and thereafter undergo chain extension polymerization withfurther of said monomer, and (c) allowing the polymerization to proceedsubstantially to completion, wherein step (b) is performed by slowlyadding the monomer at a controlled rate to a quantity of the adduct insolution over a period of several hours.

The rate of addition is preferably such that the catalyst is always instoichiometric excess over the monomer. Advantageously the rate of thepolymerization of the monomer in the presence of the catalyst is firstdetermined and the rate of addition is set such that it is slower thanthis so that the catalyst remains in stoichiometric excess.

Whereas the prior art process of Manser uses the initial stoichiometryand the reaction temperature to provide molecular weight control thepresent process adds monomer at a rate such that the ratio of themonomer to the catalyst does not exceed levels known to produce cyclicoligomerisation. On completion of monomer addition the polymerization isallowed to continue for a time, typically several hours, for examplethree hours.

The monomer is added over a period of several hours, preferably over 12hours or more, more preferably over 16 hours or more and particularlyover 18 hours or more, with a maximum preferred time of 50 hours. It hasbeen found that by slowly adding the monomer to the reaction mixture, anadvantageous increase in molecular weight in the product is observedbut, unlike the process described in U.S. Pat. No. 4393199, without anassociated increase in polydispersivity.

The controlled addition of monomer in the process of the invention alsoaffects the level of impurities in the product. If a bulk reaction isused to polymerise NIMMO as in the DTIC report about 25% of the productwill comprise impurity other than the desired polymer as measured by NMRand gel permeation chromatography. Using the method of the presentinvention the impurities are reduced to 15% with the possibility of 5%being achieved if addition is carried out over a period of over 18hours.

The effect of impurities on the polymer properties is very dependent onthe nature of the impurity present. Generally oligomers tend to act asplasticisers and lower the glass transition temperature (T_(g)) ofcured, estomeric products of the reaction between polyethers of thistype and suitable curing agents (for example, isocyanates), and quiteoften oligomer impurities can also adversely affect the cure. Smallmolecule impurities normally give a less favourable result since theycan undergo reactions which rapidly degrade the polymer. This isparticularly important with the production of polymers ofnitratoalkyl-substituted oxetanes such as polyNIMMO since the monomerand other impurities degrade rapidly with time and autocatalyticallydegrade the polymer.

The type of impurity produced in the known batch reaction tends tocomprise of unacceptably high levels of unreacted monomers and othersmall molecule products which can represent up to 30% of the isolatedproducts with about 20% being oligomers. By contrast, the presentprocess when applied to polyNIMMO improves on this purity significantly,providing a level of 5% impurities comprising of about 4% highermolecular weight oligomer impurities, which have less effect on thecharacteristics of the product, and only about 1% small moleculeimpurities.

The catalyst used in the process of the present invention is preferablyborontrifluoride etherate and is used in conjunction with diols ortriols as a preinitiator precursor but many others may be employed, forexample AgPF₆ or AgSbF₆ may be used with organic bromides (e.g., m- orp-xylilene dibromide); or HBF₄ etherate may also be used with diols andtriols. All catalysts are used in anhydrous form and nitrogen atmosphereis advantageously employed in the reaction vessel to maintain this. Itis a feature of the reaction that difunctional or trifunctional polymersmay be produced by employing di- or trifunctional agents in thepreinitiator (e.g., diols or triols or bromides.)

The ratio of the initiator components has also been found to affect themolecular weight of the product. Where a preinitiator is used care mustbe taken not to have an excess of its functional groups over thecatalyst moieties or termination of the polymer will occur prematurely.The preinitiator precursor may be a diol or triol, in which case themolar ratio of hydroxy groups in the diol or triol to catalyst ispreferably from 1:0.75 to 1:2.5, preferably from 1:1 to 1:1.5. With theborontrifluoride etherate: butan-1,4-diol system an excess of theborontrifluoride over the diol of in the molar ratio of at least 1.5:1,preferably at least 2:1, is essential to avoid premature termination ofthe polymerization reaction. The higher the ratio of borontrifluride todiol the higher the molecular weight of the product. Thus by increasingthe borontrifluoride: diol ratio from 2:1 to 5:1 (with 3:1 being thepreferred upper limit) a proportional increase in molecular weightensues.

The temperature of the reaction affects the polydispersity of theproduct such that operation at, for example, 20° C. will give a broadrange of polyNIMMO molecular weights whereas -20° C. will give a muchnarrower range. Due to the very low impractical polymerization rateachieved at -20° C. which would necessitate extremely slow rates ofmonomer addition, it is preferred to employ a temperature of 0° C. whichgives a sufficiently narrow weight range with a good polymerizationrate.

Generally both the monomer and the catalyst are used as solutions in asuitable solvent. The solvent used affects the reaction such theincreasing polarity increases side reactions. Hydrocarbon solvents suchas toluene give the best results with regard to purity but the preferredsolvent for a combination of good rate with good purity is a halogenatedhydrocarbon, preferably dichloromethane. Typically 20% w/w monomer andcatalyst solutions are used although other concentrations of up to 50%may be used.

The present polymerization is preferably performed in an apparatuscomprising a reaction vessel provided with a monomer feed line includinga controlling device for adjusting the rate of addition of monomer suchthat it may be added continuously at a rate such that the ratio ofcatalyst to monomer is at a controlled level at any one time. Thecontrol device preferably acts to achieve a pumped flow of monomer intothe vessel.

It is preferred to achieve the addition of the monomer in this apparatusby use of a pump acting upon the feed line from a monomer supply to thereaction vessel. Preferably the pump is electrically powered for ease ofcontrol.

PROTOCOL

A typically protocol for a polymerization according to the presentinvention involves the cooling of the reaction vessel at 0° C. undernitrogen gas and injecting the preinitiator precursor and solvent, ifused, into it. The catalyst precursor is added over several minutes andthe mixture stirred for about one hour to form the active adduct. Forthe butan-1,4-diol system three times its volume of dichloromethanesolvent is used and the diol forms a precipitate in this. Theborontrifluoride etherate is added to the vessel with stirring over fiveminutes and the stirring continued over a period of one hour to dissolvethe diol.

Preferably the monomer is added to the stirred reaction mixture over aperiod predicted to provide the product with the molecular weightdesired via use of a pump in the feed line, e.g., a peristaltic pumpacting on a flexible tube. The reaction is allowed to continue for afurther period of several hours, typically three or four, beforetermination with an excess of brine. The organic layer is then washedwith an aqueous base e.g., sodium hydrogen carbonate and then withwater, then separated polymer product is isolated by drying the organiclayer over calcium chloride and optionally mixing it with methanol toprecipitate the polymer before evaporation and drying in a vacuum oven.(Methanol precipitation is not necessary with the present invention butmay be employed if desired.)

The process of the present invention will now be illustrated by way ofexample only with reference to the following synthesis examples.

EXAMPLES EXAMPLES (1-6) The polymerisation of 3-nitratomethyl,3-methyloxetane(NIMMO)

SAFETY NOTE: Owing to the explosion hazard associated with this monomerthe polymerisation reaction should be undertaken in an armoured fumecupboard.

EXAMPLE 1 The preparation of difunctional polyNIMMO (crude).

The polymerisation reactor, which consisted of a 500 ml jacketed vesselequipped with a mechanical stirrer, nitrogen inlet/outlet, thermometerand serum cap was cooled from 120° C. to ambient temperatures undernitrogen. It was then connected to a cooling circulator and charged witha 25% w/v mixture of butane-1,4-diol in dichloromethane (3 g in 9 ml,0.033 mol). The reactor was then cooled under nitrogen to 0° C. and atwo fold excess of boron trifluoride etherate (9.44 g, 0.066 mol) wasthen added dropwise over a period of 10 minutes. After a delay of onehour to allow the initiatory complex to form, 20% w/v NIMMO indichloromethane (75 g NIMMO in 375 ml dichloromethane) was pumped in ata constant flow rate over a period of 18 hours. When addition wascomplete a further polymerisation period of 4 hours was allowed beforethe reaction was terminated by the addition of a 20 fold excess of brine(24 g, 1.33 mol). The polymer was then isolated by washing the organiclayer with aqueous sodium hydrogen carbonate solution, drying overcalcium chloride, then removing the solvent on a rotary evaporator. Theresultant tacky polymer was then dried at 50° C. for 60 hours in avacuum oven. Yield was 71 g (95%). The crude product was shown topossess less than 4% oligomer and 1% small molecule impurities by ¹ Hand ¹³ C NMR along with dual detector gel permeation chromatography. Themolecular weight of this polymer was M_(n) =6300, M_(w) =11000, M_(w)/M_(n) (polydispersivity)=1.74.

EXAMPLE 2 The preparation of difunctional polyNIMMO (pure).

The polymerisation reactor was set up and charged with reagents andreactants as in Example 1 and as before was run over an addition periodof 18 hours. After the further polymerisation period of 4 hours andbrine termination step the polymer was isolated by washing the organiclayer with aqueous sodium hydrogen carbonate solution, dried overcalcium chloride and then precipitated into methanol. The resultanttacky polymer was then dried at 50° C. for 60 hours in a vacuum oven.Yield was 55 g (75%). The isolated product was free from all smallmolecule and oligomeric contaminants. The molecular weight of thispolymer was M_(w) =10800, M_(n) =7500, M_(w) /M_(n) =1.44.

EXAMPLE 3 The preparation of difunctional polyNIMMO(variation).

The procedure used was exactly the same as in Example 1 except thattetrafluoroboric acid etherate (HBF₄) was used in place of the borontrifluoride etherate. There was also no need for the 1 hour wait for theinitiatory complex to form. Conversion to 91% pure crude product was 97%isolated yield. Precipitation into methanol gave contaminated with lessthan 1% oligomer in a 78% yield.

EXAMPLE 4 The preparation of difunctional polyNIMMO (variation).

The procedure was exactly the same as Example 1 except thathexafluoroantimonic acid was used in place of the boron trifluorideetherate. There was also no need for the 1 hour wait for the initiatorycomplex to form. Conversion to an 87% pure crude product was 94%isolated yield. Precipitation into methanol gave product contaminatedwith less than 2% oligomer in 78% yield.

Typical molecular weights of the polymers of Example 3 and 4 were M_(w)=9600, M_(n) =7300, M_(w) /M_(n) =1.31.

EXAMPLE 5 The preparation of trifunctional polyNIMO.

The procedure was exactly the same as that of Example 1 except thatmetriol was used instead of butane-1,4-diol, a three-fold excess ofboron trifluoride etherate was used and addition was over a 24 hourperiod. Conversion to a 94% pure crude was 93% isolated yield.Precipitation into methanol gave product contaminated with no oligomerin 75% yielded.

EXAMPLE 6 The preparation of trifunctional polyNIMMO (variation).

The procedure was exactly the same as that for Example 1 except thatmetriol was used instead of butane-1,4-diol, a three-fold excess oftetrafluoroboric acid etherate was used and addition was over a 24 hourperiod. Conversion to an 89% pure crude product was 98% isolated yield.

EXAMPLE 7 The preparation of trifunctional polyNIMMO (variation).

The procedure was exactly the same as that used in Example 1 except thatmetriol was used in place of butane-1,4-diol, a three-fold excess ofhexafluoroantimonic acid was used and addition was over a 24 hourperiod. Conversion to an 83% pure crude product was 92% isolated yield.Precipitation into methanol gave product contaminated with less than 3%oligomer in 79% yield.

Typical molecular weights of the polymers of Examples 5 to 7 were M_(w)=4900, M_(n) =3600, M_(w) /M_(n) =1.36.

EXAMPLE 8-13 The polymerisation of oxetane(trimethylene oxide). EXAMPLE8 The preparation of difunctional polyoxetane.

The procedure was exactly the same as the used in Example 1 except thatoxetane was used instead of NIMMO. The corresponding addition period was12 hours. Conversion to a 96% pure crude product was 97% isolated yield.Precipitation into methanol gave product contaminated with no oligomerin 87% yield. The molecular weight of this polymer was M_(w) =6350,M_(n) =3950, M_(w) /M_(n) =1.61.

EXAMPLE 9 The preparation of difunctional polyoxetane (variation)

The procedure was exactly the same as that used in Example 3 except thatoxetane was used instead of NIMMO. The corresponding addition period was12 hours. conversion to a 93% pure crude product was 98% isolated yield.Precipitation into methanol gave product contaminated with no oligomerin 74% yield.

EXAMPLE 10 The preparation of difunctional polyoxetane (variation)

The procedure was exactly the same as Example 4 that oxetane was usedinstead of NIMMO. The corresponding addition period was 12 hours.Conversion to an 88% crude product was 93% isolated yield. Precipitationinto methanol gave product contaminated with less than 2% oligomer in69% yield.

Typical molecular weights of the polymers of Examples 9 and 10 wereM_(w) =5400, M_(n) =3450, M_(w) /M_(n) =1.56.

EXAMPLE 11 The preparation of trifunctional polyoxetane.

The procedure was exactly the same as in Example 5 except that oxetanewas used instead of NIMMO. The corresponding addition period was 16hours. Conversion to a 92% pure crude product was 94% isolated yield.Precipitation into methanol gave product contaminated with no oligomerin 76% yield.

EXAMPLE 12 The preparation of trifunctional polyoxetane (variation)

The procedure was exactly the same as in Example 6 except that oxetanewas used instead of NIMMO. The corresponding addition period was 16hours. Conversion to an 87% pure crude product was 94% isolated yield.Precipitation into methanol gave a product contaminated with less than3% oligomer in 80% yield.

EXAMPLE 13 The preparation of trifunctional polyoxetane (variation)

The procedure was exactly the same as in Example 7 except that oxetanewas used instead of NIMMO. The corresponding addition period was 16hours. Conversion to 84% pure crude product was 91% isolated yield.Precipitation into methanol gave product contaminated with less than 4%oligomer in 79 yield.

Typical molecular weights of the polymers of Examples 11 to 13 wereM_(w) =3800, M_(n) =2640, M_(w) /M_(n) =1.44.

I claim:
 1. Process for polymerizing an oxetane monomer comprising thesteps of:(a) mixing together a substantially anhydrous catalyst capableof catalyzing the cationic polymerization of the monomer with apreinitiator precursor to yield an active adduct of the catalyst andprecursor, wherein the molar ratio of catalyst to the functional groupsof the preinitiator precursor is always at least 1:1; (b) adding themonomer to a quantity of the adduct in solution over a period of atleast 12 hours to cause the adduct to form an initiating species withthe monomer and thereafter undergo chain extension polymerization withfurther monomer; and (c) allowing the polymerization to proceedsubstantially to completion; and wherein steps (b) and (c) are eachperformed at a temperature of between +20 degrees C. and -20 degrees C.2. The process according to claim 1 wherein the monomer is added to theadduct in step (b) over a period of at least 16 hours.
 3. The processaccording to claim 1 wherein the monomer is added in step (b) so thatthe catalyst is always in stoichiometric excess over the monomer.
 4. Theprocess according to claim 1 wherein the preinitiator precursor is adiol or a triol.
 5. The process according to claim 4 wherein the molarratio of hydroxy groups in the diol or triol to catalyst is from 1:0.75to 1:2.5.
 6. The process according to claim 5 wherein the molar ratio ofhydroxy groups in the diol or triol to catalyst is from 1:1 to 1:1.5. 7.The process according to claim 1 wherein the monomer is anitratoalkyl-substituted oxetane.